In October 1948 a 37-year-old Waffen-SS officer named Fritz Knöchlein was tried before a British military court in Hamburg for a particularly nasty and gratuitous war crime. It had happened eight years earlier in northern France as British forces retreated towards Dunkirk. Exhausted and out of ammunition, a company of the Royal Norfolk Regiment took refuge in a barn near a farmhouse outside the small village of Le Paradis before finally surrendering to SS forces led by Knöchlein. As the prisoners left the barn the SS forces opened fire, killing ninety-seven of them. Two survived, hidden under the pile of bodies. Eventually they ended up in German prisoner of war camps and returned to Britain at the end of the war.

There was plenty of evidence. As well as that of the two survivors and a local French farmer, there was testimony from the interrogation of captured SS members of Knöchlein’s own unit carried out at a clandestine location in London by the War Crimes Investigation Unit. Based in several adjoining houses in Kensington Palace Gardens, it had operated during the war as the Prisoner of War Interrogation Section with the same personnel. This was the London Cage.

Intelligence from newly captured POWs could prove vital. Under the 1929 Geneva Convention, POWs were required only to provide their name and rank. But skilful questioning often produced gold. The Cage interrogators concentrated on those most likely to yield high-carat information. Through the interrogation of some three thousand German POWs,